Sight devices are commonly used with firearms to provide a shooter with an aiming point. Several types of sight devices are available. For example, iron sights typically include a first sight piece positioned near the muzzle end of a firearm and a second sight piece positioned nearer to the breach end. The first and second sight pieces are positioned appropriately with respect to one another to align the firearm with a target.
Optical sights are another type of sight device and include optical components, such as lenses, and an indication of an aiming point. Typically, this indication of an aiming point is in the form of a reticle, which can have many configurations, such as dots, crosshairs, and others. Telescopic sights are a type of optical sight and include lenses that magnify the image viewed through the telescopic sight.
A reticle is typically provided in an optical sight by positioning a reticle piece, sometimes referred to as reticle glass, at an appropriate position in the optical components of the optical sight. A reticle piece includes a reticle pattern and is typically positioned at a focal plane so that it provides an in-focus reticle, when viewed by a shooter. For example, telescopic sights typically include a front focal plane and a rear focal plane, and the reticle piece can be positioned at either of those focal planes.
A reticle is a graphic image superimposed over the view seen through an optical sight. A crosshair reticle is a common type of reticle and includes a vertical segment and a horizontal segment which intersect one another in a central region of the view seen through the optical sight. In general, the intersection of the vertical and horizontal segments provides the aiming point that a shooter aligns with a target.
For relatively close targets, the aiming point may coincide with the point that a bullet will impact. In addition, either or both of the vertical and horizontal segments can include additional markings relevant to factors relating to an appropriate aiming point. For example, a vertical segment may include graduated hashes or other marks that correspond with the amount that a bullet will drop (due to gravity) as it follows its trajectory to a distant target. For more distant targets, the aiming point provided by the intersection of the vertical and horizontal segments of the reticle may not coincide with a point of bullet impact unless the aiming point is adjusted to compensate for bullet drop. Generally, as the distance to a target increases, a shooter will have to account for the amount that a bullet will drop. The graduated hashes on the vertical segment of the reticle can assist a shooter in addressing this bullet drop.
If an optical sight is properly leveled with respect to the ground, the bullet drop will follow along the vertical segment of the reticle (assuming there is no cross-wind). If the optical sight is not properly leveled, however, such as if the firearm to which the optical sight is attached is held in a tilted orientation, then the bullet drop will not follow along the vertical segment of the reticle, and the graduated hashes on the vertical segment will not be useful to the shooter.
There is a need, therefore, for devices that provide an indication of whether an optical sight is properly leveled with respect to the ground. Various external devices have been used, but these require the shooter to move his eye away from the view through the optical sight in order to check or confirm the level of the firearm. The shooter must then return his view to the optical sight, but can no longer with the external device. Various electronic devices have been proposed which provide an internally viewed level indicator, but these require a power source and significantly increase the cost of the optical sight. Internal mechanical devices have to be trued to the reticle and are subject to becoming misaligned.